The present invention relates generally to fluid infusion systems and more particularly is directed to a method and apparatus for accurately detecting small changes in tubing dimensions to prevent high pressure infusion into the fluid infusion system.
The infusion of fluids into the human body is usually accomplished by means of an administration set in conjunction with metering apparatus which controls the rate of flow of fluid through the set. Peristaltic-type pumps, which function by repetitively compressing and expanding a section of tubing, have proven particularly attractive for use in metering apparatus since they do not introduce the possibility of leakage or contamination into the system, while providing positive control fo fluid flow through the system.
One form of peristaltic-type pump which is particularly well adapted for infusion applications is described in U.S. Pat. No. 4,155,362. Basically, this pump construction includes individually spring-biased rollers in the pump rotor which provide a uniform compression force, and a spring-biased plunger which restricts the lumen of the administration set downline of the pump rotor to provide a back pressure against which the pump must work. This prevents the release of dissolved gas in the tubing section, assists in restoring the tubing to its original shape following compression by the pump, and prevents uncontrolled gravity flow in the event of pump failure.
A potential problem with this and other types of infusion systems relates to detecting pressure increases downline of the pump or pump restrictor in order to avoid patient harm which may occur as a consequence of excessive fluid pressure. Some prior systems have included sensing means with the pumping system, but these are ineffective downline of pumping systems which are isolated from downline pressure variations by some type of restrictor.
One prior attempt at solving this problem was the use of a push rod microswitch system. As the tubing would swell from the increase in pressure caused by an occlusion downline of the pump, the rod would activate the microswitch and cause an alarm and/or stop the pump. A basic problem with this type of microswitch system is that the calibration of the system requires that the switch respond at a specific position. Therefore, anything which affects the alignment and contact pressure between the push rod and the tubing has the capability of changing significantly the internal pressure which is required to cause the tubing to swell sufficiently in order to trigger the microswitch. Further, the stability of the mechanical system and the microswitch has to be accurate enough to measure as little as 0.001 inch travel. Variations such as spring fatigue and friction variations introduce other deviations in the operation. The cross sectional shape that the tubing has at the time of insertion into the pump can vary from circular to extremely elliptical. This can result in differences as large as two to one in the dimension which is sensed by the push rod.
Further, the tubing itself will vary in wall thickness and inside diameter by about 0.01 inch. In addition the tubing material itself will vary in the pressure it takes for it to swell a certain amount. The operating temperature range and age of the tubing will cause variations in the amount of swelling per unit of pressure. These many variations can cause the actual pressure at which the push rod system will alarm to vary widely from infusion set to infusion set and with each insertion of the same tubing into the pump.